CrB2 -NbB2 ceramics materials

ABSTRACT

Ceramic materials based on solid solutions of chromium diboride (CrB 2 )nd niobium diboride (NbB 2 ).

BACKGROUND

This invention relates to ceramics and more particularly to ceramics based on metal borides.

Previous studies on diboride materials have focused primarily on group VI metal diborides. Ceramics based on zirconium diboride (ZrB₂) and Hafnium diboride (HfB₂) were developed through the 1960s by the U.S. Air Force for advanced hypersonic vehicle leading edges. While these materials are useful it would be desirable to produce new, improved ceramic materials.

SUMMARY OF THE INVENTION

An object of this invention is to provide a new ceramic material.

Another object of this invention is to provide a new ceramic material that is very hard.

A further object of this invention is to provide a new ceramic material that is resistant to oxidation.

These and other objects of this invention are achieved by providing a ceramic material that is based on a solid solution comprising from more than zero to less than 100 mole percent CrB₂ with the remainder in the solid solution being NbB₂.

BRIEF DESCRIPTION OF THE FIGURES

A more complete understanding of the invention and many of the attendant advantages thereto will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying figures wherein:

FIG. 1 is a plot of hardness versus composition for ceramic materials based on CrB₂ -NbB₂ solid solutions, and

FIG. 2 is a plot of weight change due to oxidation versus composition for ceramic materials based on CrB₂ -NbB₂ solid solutions.

DESCRIPTION OF THE INVENTION

The ceramic materials of this invention are based on solid solutions of chromium diboride (CrB₂) and niobium diboride (NbB₂). The CrB₂ -NbB₂ solid solution comprises from more than zero to less than 100 mole percent CrB₂ with the remainder in the solid solution being NbB₂. FIG. 1 is a plot of the hardness of the ceramic material versus its composition (mole % CrB₂). FIG. 1 shows that the addition of even a small amount of CrB₂ to NbB₂ increases the hardness and similarly the addition of NbB₂ to CrB₂ increases the hardness of the ceramic material. For hardness, the ceramic material is based on a CrB₂ -NbB₂ solid solution having a composition of preferably from about 20 to about 85, more preferably from 60 to 85, and still more preferably from 75 to 80 mole percent of CrB₂ with the remainder in the solid solution being NbB₂. FIG. 2 shows the change in weight due to oxidation in air in furnaces at 1000° C., 1100° C., 1200° C., and 1300° C. for various CrB₂ -NbB₂ solid solution ceramic compositions. Samples which have significant changes in weight due to oxidation also have noticeable changes in appearance such as changes in color, size, shape, density, strength, etc. For applications where the ceramic material is not heated above 1000° C., oxidation will not be a problem if the CrB₂ -NbB₂ solid solution ceramic material has a composition of from 10 to less than 100 mole percent CrB₂ with the remain in the solid solution being NbB₂. In that case the CrB₂ -NbB₂ solid solution composition is selected for hardness. For oxidation resistance when the CrB₂ -NbB₂ solid solution ceramic material is heated above 1000° C., a CrB₂ -NbB₂ solid solution having a composition of preferably from about 15 to about 70 and more preferably from 20 to about 60 mole percent of CrB₂ with the remainder in the solid solution being NbB₂ is used. When the ceramic material is heated above 1000° C. during use, the preferred embodiment is a ceramic material based on a solid solution of about 60 mole percent CrB₂ with the remainder in the solution being NbB₂. This composition provides maximum oxidation resistance with good hardness. When the ceramic material is not heated above 1000° C. during use, the preferred embodiment is a ceramic material based on a solid solution of about 80 mole percent CrB₂ with the remainder in the solid solution being NbB₂. This composition provides maximum hardness.

The process of making the CrB₂ -NbB₂ solid solution ceramic material begins with the preparation of an intimate mixture of CrB₂ and NbB₂ powders. Commercially available CrB₂ and NbB₂ powders having a particle size and purity suitable for ceramic processing are used. In the examples, CrB₂ powders (99.5% purity, -325 mesh) and NbB₂ powder (99.5% purity, -325 mesh) from CERAC INC. was used. The intimate mixture is formed by convention means. In the examples, a slurry of the desire proportions of CrB₂ powder, NbB₂ powder, and a volatile organic solvent (methanol, acetone, etc.) was formed and thoroughly mix. The CrB₂ fNbB₂ powder mixture was dried and passed through a 500 micron screen three times.

The intimate CrB_(2/) NbB₂ powder mixture is then converted into a CrB₂ -NbB₂ solid solution ceramic body by hot pressing. For simple geometric shapes conventional die hot pressing techniques are used. More complex ceramic bodies can be prepared by conventional hot isostatic pressing (HIP). The hot pressing must be done in a noble gas (argon, helium, neon, etc.) atmosphere or in a vacuum.

Samples of CrB₂ -NbB₂ solid solution ceramic materials which were prepared using a graphite die are listed in table 1. Table 1 gives the compositions of the intimate mixtures of CrB₂ and NbB₂ powders used, as well as the process temperature, pressure, and time for each composition.

                  TABLE 1                                                          ______________________________________                                         COMPOSITION     PROCESS CONDITIONS                                             Mole %   Weight %   Temperature                                                                               Pressure                                                                              Time                                     NbB.sub.2                                                                            CrB.sub.2                                                                             NbB.sub.2                                                                              CrB.sub.2                                                                           (°C.)                                                                            (MPa)  (min.)                               ______________________________________                                         100    0     100     0    2100     20     30                                   95     5     96.76   3.27 2100     30     30                                   90    10     93.33   6.66 2000     30     30                                   80    20     86.15   13.84                                                                               2000     30     30                                   60    40     70.00   30.00                                                                               1900     30     30                                   40    60     50.91   49.09                                                                               1900     30     30                                   20    80     28.01   72.00                                                                               1900     30     30                                   10    90     14.74   85.35                                                                               1900     30     30                                    5    95     7.57    92.43                                                                               1900     30     30                                    0    100    0       100  1900     20     30                                   ______________________________________                                    

Thus, the CrB₂ -NbB₂ solid solution ceramic materials can be prepare by hot die pressing the intimate CrB_(2/) NbB₂ powder mixture for 30 minutes at a pressure of 30 MPa and at a temperature of 2100° C. if the CrB₂ content is less than 5 mole percent, 2000° C. if the CrB₂ content is from 5 to less than 20 mole percent, or 1900° C. if the CrB₂ content is from 20 to less than 100 weight percent in the solid solution. Hot die pressing for 30 minutes at 2100° C. and 30 MPa should produce a good CrB₂ -NbB₂ solid solution ceramic material for any composition. Again the hot pressing must be done in a noble gas (argon, helium, neon, etc.) or in a vacuum. The pressure used to prepare these ceramic materials can be varied. Pressures of from 10 MPa to 40 MPa (limited by the graphite die strength) were used to produce the CrB₂ -NbB₂ solid solution ceramic materials.

The ceramic process temperature should preferably be from about 1900° C. to 2100° C. as illustrated by Table 1. The melting point (˜2200° C.) of CrB₂ limits the upper end of the ceramic process temperature range. The process temperature should be at least 1900° C. in order to have a practical process time. On the other hand, as the temperature is decreased, the time required to form solid solutions increases exponentionally. For example, the formation of continuous solid solutions at 1600°, 1500°, and 1400° C. was accomplished after 10, 72, and 228 hours of heat treatment, respectively, as compared to 30 minutes at 1900° C.

CrB₂ -NbB₂ solid solution ceramic materials prepared by hot pressing using graphite die and the process conditions given in Table 1 were tested for hardness. The results are plotted in FIG. 1. Each cross (+) is an average of the hardness measurements for that composition. The squares above and below the average curve show the range of the test results for each composition. In the preferred compositional ranges, even the lowest hardness measurement exceeds the hardness of pure NbB₂ and of pure CrB₂. Moreover, over the entire solid solution compositional range the lowest hardness measurements exceed what can be expected from the rule of mixtures. Also the pattern of hardness versus composition is the same for the lowest and highest measurements (as well as the averages).

Vickers hardness tests were conducted using LECO V-100-C Series Vickers Macrohardness Tester to both determine hardness and estimate the fracture toughness of the materials. Indents were made at a 20 kg load for 15 seconds on fully dense samples which were polished through a 6 μm diamond finish. For comparison, the hardness was also measured at a 2 kg load. The hardness was calculated using the formula: H=1.8544L/d², where H is the hardness in kg/mm², L is the load in kg, and d is the diagonal in mm. Typically, hardness values are the average of at least five indentations.

The furnace oxidation experiments (see FIG. 2) showed very poor oxidation resistance of both NbB₂ and CrB₂ ceramics above 1000° C. FIG. 2 shows the weight change ×10⁴ (g/cm²) for the CrB₂ -NbB₂ solid solution ceramic materials of table 1 when heated at 1000° C. (small squares), 1100° C. (crosses, +), 1200° C. (stars, * ), and 1300° C. (large solid squares) in air for 2 hours. The NbB₂ samples had a thick, porous, non-adherent coating, containing Nb₃ BO₉ and small amount of B₂ O₃, which could not provide effective oxidation protection. The thickness of the oxidation product increased with increasing temperature and/or holding time. The weight change data probably represent both a gain in weight from the formation of niobium oxide and B₂ O₃ and a loss in weight due to the evaporation of residual B₂ O₃ left after reaction between the oxides. In this case, the evaporation of B₂ O₃ probably controls the overall oxidation process of NbB₂ ceramics.

The oxidized CrB₂ samples were coated with well adherent layer of Cr₂ O₃ and CrBO₃ which protected the sample even after 100 hours hold at 1000° C. At higher temperature, gradual softening and melting of CrBO₃ with simultaneous formation of gaseous B₂ O₃ resulted in the foaming of the oxidation product and loss of protecting capabilities. Ceramics of intermediate compositions showed a dramatic improvement in oxidation behavior compared to the end members. Compositions containing between 20 and 60% CrB₂ showed practically no oxidation at 1200° C. The samples were coated with very thin adherent layer consisting of Nb₃ BO₉ and B₂ O₃ bonded with the high melting point (1700° C.) CrNbO₄.

Obviously numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described herein. 

What is claimed is:
 1. A material which is a ceramic based on a solid solution comprising from more than zero to less than 100 mole percent CrB₂ with NbB₂ being the remainder in the solid solution.
 2. The material of claim 1 which is a ceramic based on a solid solution comprising from about 20 to about 85 mole percent of CrB₂ with NbB₂ being the remainder in the solid solution.
 3. The material of claim 2 which is a ceramic based on a solid solution comprising from 60 to 85 mole percent CrB₂ with NbB₂ being the remainder in the solid solution.
 4. The material of claim 3 which is a ceramic based on a solid solution comprising from 75 to 80 mole percent of CrB₂ with NbB₂ being the remainder in the solid solution.
 5. The material of claim 1 which is a ceramic based on a solid solution comprising about 80 mole percent CrB₂ with NbB₂ being the remainder in the solid solution.
 6. The material of claim 1 which is a ceramic based on a solid solution comprising from about 15 to about 70 mole percent CrB₂ with NbB₂ being the remainder in the solid solution.
 7. The material of claim 6 which is a ceramic based on a solid solution comprising from 20 to 60 mole percent CrB₂ with NbB₂ being the remainder in the solid solution.
 8. The material of claim 1 which is a ceramic based on a solid solution comprising about 60 mole percent CrB₂ with NbB₂ being the remainder in the solid solution. 